The centre-half suffered sprained medial ligaments during Saturday's 1-0 win at Charlton - Millwall's first clean sheet of the season.
Lions boss Steve Lomas told the club website: "It's obviously a blow for us and for Mark at a time when we appear to be turning a corner.
"Fortunately surgery doesn't appear to be necessary but we are resigned to being without him for the next few weeks."
- Sports & Recreation
- Danny Shittu